A typical air-conditioning apparatus has a refrigerant circuit configuration in which a compressor, a four-way valve, an outdoor heat exchanger, an electronic expansion valve, and an indoor heat exchanger are connected. The compressor, the four-way valve, and the outdoor heat exchanger are accommodated in an outdoor unit together with an outdoor unit-side fan for sending air to the outdoor heat exchanger. The electronic expansion valve and the indoor heat exchanger are accommodated in an indoor unit together with an indoor unit-side fan for sending air to the indoor heat exchanger. The outdoor unit and the indoor unit are connected to each other with a plurality of extension pipes.
Further, the outdoor unit includes a high-pressure sensor for detecting a discharge pressure of the compressor, a low-pressure sensor for detecting a suction pressure of the compressor, and a discharge temperature sensor for detecting a discharge temperature of the compressor. The indoor unit includes an indoor heat exchanger outlet temperature sensor for detecting a temperature of refrigerant that has passed through the indoor heat exchanger during heating operation. A controller controls the compressor, the four-way valve, the electronic expansion valve, the outdoor-side fan, and the indoor-side fan based on information acquired from the above-mentioned sensors, for example.
In the above-mentioned refrigerant circuit, during the heating operation, there is formed a flow passage for causing the high-pressure refrigerant discharged from the compressor to flow into the indoor heat exchanger. With this, during the heating operation, the indoor heat exchanger serves as a condenser, and the outdoor heat exchanger serves as an evaporator.
Patent Literature 1 discloses an air-conditioning apparatus configured to form a refrigeration cycle by sequentially connecting a low stage-side compressor capable of adjusting a rotation speed, a high stage-side compressor capable of adjusting a rotation speed independently of the low stage-side compressor, a condenser, a first pressure reducing device, and an evaporator. Between the condenser and the first pressure reducing device of this air-conditioning apparatus, an intercooler (internal heat exchanger) is arranged. Part of the refrigerant flowing out from the condenser becomes a branched flow branched from a main-stream refrigerant, and is depressurized to an intermediate pressure through a second pressure reducing device. The depressurized branched flow exchanges heat with the main-stream refrigerant at the intercooler, and then flows into the suction side of the high stage-side compressor.
Further, in Patent Literature 2, there is disclosed an air-conditioning apparatus including a refrigeration cycle in which an injection compressor, a condenser, a first pressure reducing device, and an evaporator are sequentially and annularly connected, and an injection circuit branched at a branching portion between the condenser and the first pressure reducing device, for injecting the refrigerant to the injection compressor through a second pressure reducing device. This air-conditioning apparatus includes an internal heat exchanger for exchanging heat between the refrigerant of the injection circuit, which is depressurized by the second pressure reducing device, and the refrigerant of the refrigeration cycle, which flows between the branching portion and the first pressure reducing device.